BALANCE FUNCTIONS: Multiplicity and transverse momentum dependence of the charge dependent correlations in ALICE

Abstract

The measurement of charge-dependent correlations between positively and negatively charged particles as a function of pseudorapidity and azimuthal angle, known as the \emph{balance functions}, provide insight to the properties of matter created in high-energy collisions. The balance functions are argued to probe the creation time of the particles and are also sensitive to the collective motion of the system. In this thesis, I present the results of the measured balance functions in p--Pb collisions at $\sqrt{s_{NN}} = 5.02~TeV$ obtained with the ALICE detector at the LHC. The results are compared with balance functions measured in pp and Pb--Pb collisions at $\sqrt{s}=7~TeV$ and $\sqrt{s_{NN}} = 2.76~TeV$, respectively. The width of the balance functions in both pseudorapidity and azimuthal angle for non-identified charged particles decreases with increasing multiplicity in all three systems, for particles with low transverse momentum value $(p_{T} < 2~GeV/c)$. For higher values of transverse momentum the balance functions become narrower and exhibit no multiplicity dependence. The experimental findings are compared to different models (PYTHIA8, DPMJET, HIJING and AMPT). The comparison with models seems to indicate that for the Pb--Pb system $\Delta\varphi$ is determined mainly by radial flow, while $\Delta\eta$ is less sensitive. In addition, the color reconnection mechanism emerged as a necessary ingredient in understanding the multiplicity dependence observed in the width of the balance function for small systems.